Numerous industrial processes require separating one or more acid gases such as CO.sub.2 and H.sub.2 S from gaseous mixtures containing such acid gases and additional non-acid gas components such as CH.sub.4 and H.sub.2. Such processes include separating carbon dioxide from hydrogen gas in hydrogen synthesis plants and removing H.sub.2 S and CO.sub.2 during the cleanup of natural gas. Numerous types of membranes have been evaluated for use in such processes taking into account economic and energy consumption considerations.
Preferred membranes for separating acid gases from gaseous mixtures are those which permeate the desired acid gases at a preferential rate over non-acid gas components such as hydrogen gas and methane in the process stream to be treated and which are highly permeable with respect to such acid gases. However, such membranes are relatively rare and sometimes have limited utility because the membranes exhibit insufficient acid gas permeability.
U.S. Pat. No. 4,500,667 discloses gas separation membranes comprising an organic polymer-inorganic compound blend which is prepared by admixing an organic polymer such as poly(vinyl alcohol) with a heteropoly acid or salt thereof such as dodecamolybdophosphoric acid in a mutually miscible solvent. After allowing the mixture to react for a period of time sufficient to form a blend, the solution is cast on an appropriate casting surface, the solvent is evaporated and the desired membrane is recovered. The membranes are suitable for separating water from a water-containing hydrogen stream.
U.S. Pat. No. 4,780,114 discloses membranes which selectively permeate acid gases such as CO.sub.2 or H.sub.2 S over non-acid gas components. The membranes comprise a thin film of a molten salt hydrate which may be immobilized within the pores of a thin, porous inert support or alternatively, may be encapsulated in a non-porous, gas permeable polymer such as poly(trimethylsilylpropyne), polymer blends or silicone rubber. The term, molten salt hydrate, refers to a salt, which upon heating, melts to yield a liquid system which contains bound water. Molten salt hydrates are represented by the formula A.sub.x B.sub.y.nH.sub.2 O wherein A and B are ionic species of opposite charge and n represents the number of moles of bound water per mole of salt. Representative molten salt hydrates include tetramethylammonium fluoride tetrahydrate and tetramethylammonium acetate tetrahydrate.
U.S. Pat. No. 4,913,818 discloses a process for removing water vapor from a gas/vapor mixture such as a water-alcohol mixture, which utilizes a regenerated cellulose membrane which is impregnated with a hygroscopic electrolyte salt. The electrolyte is desirably a salt of an alkali metal, an alkaline earth metal or a transition metal wherein the anion is a chloride, bromide, fluoride, sulphate or nitrate. Representative salts include LiBr, KCl, MgCl.sub.2, CaCl.sub.2, SrSO.sub.4 and NaNO.sub.3. A cellulose membrane impregnated with LiBr provided a 2.5-fold increase in water vapor flux over the same cellulose membrane containing no salt.
Jansen and coworkers, (Proc. Int. Conf. Pervaporation Processes Chem. Ind., 3rd, 338-341, 1988) disclose cellulose and poly(vinyl alcohol) (PVOH) membranes which are impregnated with CsF. The CsF-impregnated PVOH membrane provided an approximate doubling of water vapor flux although water/alcohol selectivity decreased by an unspecified amount. Higher water vapor fluxes were obtained by repeatedly impregnating the membrane with CsF. The reference does not teach or suggest that the disclosed membranes permeate carbon dioxide.
U.S. Pat. No. 4,973,456 discloses a process for reversibly absorbing acid gases such as CO.sub.2, H.sub.2 S, SO.sub.2 and HCN present in a gaseous mixture wherein the gaseous mixture is contacted with a hydrate salt represented by the formula A.sub.x.sup.m+ B.sub.y.sup.n-.rH.sub.2 O wherein A.sup.m+ is a cation, B.sup.n- is the conjugate base of a weak acid having a pK.sub.a corresponding to an ionization constant of the acid greater than 3 as measured in dilute aqueous solution, m and n are independently selected integers from 1-4, x and y are integers such that the ratio of x to y provides a neutral salt and r is any number greater than zero up to the maximum number of moles of water which can be bound to the salt. Representative salt hydrates include tetramethylammonium fluoride tetrahydrate, tetramethylammonium acetate tetrahydrate and cesium fluoride.
U.S. Pat. No. 5,062,866 discloses a process for separating an unsaturated hydrocarbon from a feed stream containing such unsaturated hydrocarbon wherein the feed stream is contacted with a membrane comprising a blend of hydrophilic polymer and a hydrophilic alkali metal salt and metals which are reactive with respect to the desired unsaturated hydrocarbon. Suitable polymers include polyvinyl alcohol, polyvinyl acetate, sulfonyl-containing polymers polyvinylpyrrolidone and the like. Suitable metal salts include silver nitrate.
U.S. Pat. No. 5,336,298, assigned to Air Products and Chemicals, Inc., Allentown, Pa., teaches a process for separating acid gases from a gaseous mixture containing acid gas and at least one non-acid gas component wherein the gaseous mixture is brought into contact with a multilayer composite membrane comprising an essentially non-selective polymeric support layer and a separating layer comprising a polyelectrolyte polymer which contains cationic groups which are electrostatically associated with anions for which the pK.sub.a of the conjugate acid is greater than 3. The multilayer composite membrane selectively permeates acid gas thereby removing the same from the gaseous mixture. Suitable polyelectrolyte polymers include poly(diallyldimethylammonium fluoride), poly(diallyldimethylammonium acetate), poly(vinylbenzyltrimethylammonium fluoride) and poly(vinylbenzylammonium acetate).
Industry is searching for improved membranes for separating acid gases from acid-gas containing gaseous mixtures wherein the membrane would desirably exhibit substantially improved permeability to the acid gas to be separated from the feedstream without sacrificing selectivity for the component to be passed through the membrane.